When using an electrostatic marking system, a uniform electrostatic charge is placed upon a reusable photoconductive surface. The charged photoconductive surface is then exposed to a light image of an original to selectively dissipate the charge to form a latent electrostatic image of the original on the photoreceptor. The latent image is developed by depositing finely divided marking and charged particles (toner) upon the photoreceptor surface. The charged toner is electrostatically attached to the latent electrostatic image areas to create a visible replica of the original. The toned developed image is then transferred from the photoconductor surface to a final image support material, such as paper, and the toner image is fixed thereto by heat and pressure to form a permanent copy corresponding to the original.
In a typical electrostatic system, a photoreceptor surface is generally arranged to move in an endless path through the various processing stations of the Xerographic process. The photoconductive or photoreceptor surface is generally reusable whereby the toner image is transferred to the final support material, and the surface of the photoreceptor is prepared to be used once again for another reproduction of an original. In this endless path, several stations of corona charging are traversed. In known electrostatic copy processes, as those above noted, a number of electrostatic charging devices are used at various stations around the photoreceptor drum or belt. For example, at the following station: charge, recharge, pre-transfer, transfer, detack and preclean. These charging stations may involve a single corona device or multiple corona devices. Multiple corona device systems can be of a single type or a combination of different types of corona generating devices.
Many varied charging means are used for applying an electrostatic charge to the photosensitive member such as corona generating pins (Pin Corotron), corona generating wires (Corotron) or corona generating glass coated wire (Discorotron), for some examples. These devices can also be covered with a grid to further assist in generating a more uniform charge known as Pin Scorotron, Scorotron or Discorotron, respectively. These charging devices can be used as a single device or in a multiple device configuration utilizing any combination of devices mentioned. In high quality xerographic reproduction systems, a uniform charge is the foundation for production of a high quality output print.
Generally, the structure of a discorotron uses a thin, glass-coated wire mounted in an elongated U-shaped housing between two insulating anchors called “insulators”. These support the wire in the U-shaped housing in a spring tensioned manner in a singular plane. These insulators also position the wire relative to a known ground plane also known as a shield within the discorotron. The U-shaped housing can be made from aluminum and functions as the shield or can be made from plastic and is in one embodiment an elongated U-shaped structure in which case a separate shield must be provided. The corona generating electrode is typically highly conductive and when in use is placed in close approximation to the surface to be charged. Obviously, the uniform charging of a photoreceptor is necessary for the proper operation of a xerographic machine.
A by-product of corona charging devices are several gasses (most notably NOx and ozone) which are referred to in this discussion as “effluents”. Effluents must be managed in today's machines for many reasons which will be discussed in this disclosure. This management is usually through some type of air extraction and filtering system. The effluents can interact with the surrounding atmosphere, which may include organic compounds like morpholine, and with the photoreceptor itself to produce substantial negative charging effects on the photoreceptor and the resulting copy. These are sometimes called lateral charge migration (LCM) and/or parking deletion. This can cause the output of a printed copy to appear blurry or have areas where the image is entirely missing or deleted.
Nitric oxide deletions and other effluents have been a pervasive and persistent problem in these electrostatic copying systems. The shield embodiments of this invention are simple and effective ways to minimize these problems.
There are presently three forms of charging devices: corotrons, scorotrons and discorotrons. All will be referred to in this disclosure as “corotrons” or a source of “corona” discharge. The charging devices use high voltages to create a corona. This corona can be thought of as a collection of ions (charged atoms or molecules) in a local area. In most cases, the corona is influenced to move towards the desired target by the opposite charge on a screen or grid-type device.
The different names of the charge device or corotrons denote different configurations. Corotrons are simply bare wires. A high DC potential is placed on the corotron to create the corona. To charge photoreceptors to a positive voltage, a large positive DC voltage is placed on the corotron wire. To charge negatively, a negative potential is placed on the wire. Discorotrons are a wire device also. In this case, the wire is coated with a thin film of dielectric glass. Discorotrons have an alternating voltage placed on them to create both positive and negative ions. A screen or grid with a DC bias directs the discorotron's charge toward the photoreceptor. The grid voltage determines the polarity and amplitude of the charge placed on the photoreceptor.
An important consideration is that there are many ways to charge photoreceptors. Some ways have a propensity for problems to occur while others have less of an issue. In relation to nitric oxide deletions, the AC devices (discorotrons) and the negative DC devices have a higher probability of deletion problems.
The charge device is the originator of the nitric oxide parking deletion (or, for sake of clarity, deletion). The deletion process begins with the production of corona in normal atmosphere. Corona is a “cloud” of charged ions. Different types of corona contain different ions, H+ and N4+ are the major positive ions for both AC and DC devices. The negative ions NO3− and O3− (ozone) are the major ions in negative DC discharge and AC with airflow. AC devices (discorotrons) also produce the following negative ions: O−, OH−, O2−, NO2−, CO3−.
The ozone (O3) and NOx (NO and NO2) occur in relatively large amounts. These compounds are also very chemically reactive. NOx is known as Oxides of Nitrogen. While both gasses and morpholine can contribute to the deletion problem, NOx has been cited as the main culprit, hence the reference in literature and studies to “Nitric Oxide Deletion”.
Recent experiments show that the NOx output from a discorotron operated at nominal voltage is entirely NO2. Charge device NO2 output is attributed to the presence of ozone in the charge device area. Ozone oxidizes NO to NO2.
The oxidation of NO to NO2 produces one photon of light at about 1200 nm. This occurs in about 20% of the oxidized NO2. As the molecule decays to a stable state, a photon is emitted with the peak excitation of 1200 nm. This is the basis for a Chemilluenesence Nitric Oxide detector sometimes used in the prior art to measure effluents.
Photoreceptors have been shown to be very sensitive to nitric acid-type compounds (HNO3 and HNO2). The nitric acid attacks certain molecules in the transport layer of the photoreceptor rendering them too conductive. This conductivity allows any developed charge on the photoreceptor to leak to ground in the area of the attack or spread in what is sometimes (mistakenly) called lateral charge migration. Lateral charge migration is a separate issue involving the deposit of conductive salts on the photoreceptor through the interaction of corona and atmospheric contaminants, such as morpholine. In Nitric Oxide deletions, in the worst cases, areas near the acid attack appear blank on a copy because toner is not developed to the photoreceptor in those areas. In lesser extent cases, the problem manifests itself as a blurring of the image. Some volatile organic compounds, such as morpholine and organic nitrates are effluents also detrimental to the photoreceptor.
Nitric oxide deletions are often termed parking deletions. This nomenclature arises from the way in which nitric oxide deletions are most prevalent. When charging devices are run for a long period of time (during a long print run) a relatively large amount of NOx and O3 (as above indicated, collectively known as effluents) are built up. The effluents become adsorbed on the surface of nearby solids. When the machine is shut down, the photoreceptor stops rotation and becomes “parked” with a small area directly adjacent to the charge device. Over a short period of time, the adsorbed effluents are released from the charge device in a process known as out gassing. Since the photoreceptor is parked in very close proximity to the charge device, a small local area of the photoreceptor becomes damaged.
The titanium shield embodiments of the present invention provide strategies employed to combat and minimize these deletions.
It is known to use titanium to help chemically reduce effluents around a photoreceptor. (By virtue of its native oxide surface layer). It is also known to use titanium dioxide (TiO2) to remove nitric oxides from the environment via titanium dioxide coatings. See articles “Reactive Oxygen Species inhibited by Titanium Dioxide Coatings” (Suzuki et al.)) R. Suzuki, J. Muyco, J. McKitrrick, J. Frangos. J. Biomed. Mater. Res. A, 2003, Aug. 1 vol. 66 No. 2: pg. 396-402, and “Titanium Dioxide: Environmental White Knight”, L. Frazer Environmental Health Perspectives Vol. 109, No. 4, April 2001.